VFS helpers for getting/putting FDs

So that 9ns can use FD numbers from VFS space, and we don't need to
convert between the two.
9ns uses FDs from the VFS FD space

We no longer need to have struct files around just to do the translation
from FD -> 9ns chan.  We all just use the same FD.  Cleans up a lot, and
we don't need to proliferate the open_files muck.  Should be really easy
to remove the VFS/9ns hack.